Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 126
12 6 RE SIDENT~L SIBS ON GROUT
8.0 DESIGN OF TYPE IV SLABS
Design procedures for this type of slab need little elaboration,
since it is a framed slab in the engineering sense-supported with-
out contacting the soil on the site. However, some points of caution
bear repeating.
This slab is appropriate for use with any soil, since it does not
rest on soil anywhere over its entire area. It is designed in accord-
ance with conventional engineering practices, i.e., supported on
piles, piers, or footings which rest on unyielding stable soil or
rock. In areas of highly expansive soils, contact should not be
permitted with slab or grade beams; otherwise pressure sufficient
to damage the slab may result. In addition, it is advisable to pro-
vide protection-by the use of belled reinforced caissons, greased
tubes, or other means-to reduce the effect of friction on piers or
piles passing through expansive soils.
PART B: Quality Control
1.0 GENERAL
Satisfactory performance of slabs-on-ground cannot be assured by
design alone. Appropriate control of site preparation, and of qual-
ity of materials and workmanship at each step in the construction
process, is of no less importance. While the loads normally en-
countered with a residential slab-on-ground will be relatively light
as compared with those in larger multi-storied buildings, it is
nevertheless true that carefully designed house slabs can be and
are damaged through lack or inadequacy of quality control. Thus,
the quality and soundness of the finished slab are dependent to a
large extent on procedures often ignored or compromised.
Practices contained herein are offered as guides; experience
indicates that adherence will result in a finished slab which per-
forms as intended by the designer.
OCR for page 127
SUPPLEMENTARY INFORMATION 127
2 . O SITE PRE PARATION
It is vital that the soil upon which the slab is to be placed be of uni-
form density over and through the entire slab site. The method used
is secondary to the uniformity and degree of compaction achieved.
Failure to obtain uniform compaction will lead to soil settlements
and induced stresses in the slab which cannot be structurally ac-
commodated. The degree of compaction needed varies with the
plasticity index of the soil (PI), and the anticipated climatic varia-
tion, since, as has been adequately demonstrated, different soil types
react differently to changes In moisture. If proper advantage is to
be taken of this characteristic in assuring uniform soil support for
the slab, careful attention should be given to preparing the soil to
receive the slab.
The distortions of a soil mass because of heaving become in-
creasingly pronounced with increases in density. On the other
hand, distortions due to settlement as a result of loading are likely
to occur in a loose soil mass. Of these two effects, heaving can
potentially induce effects more destructive to the superstructure.
Further, the potential for heaving increases with the PI of clays.
In these circumstances, the required degree of density, on a site
expected to receive construction loads, should be as high as possi-
ble-in order to limit settlement-for soils not susceptible to heav-
ing. On the contrary, for a soil mass in relatively looser condition
that can better accommodate the potential heaving action within its
volume of voids, the degree of compaction of soils subject to heav-
ing (because of soil consistency or weather or both) should be lower
than for time-stable soils.
In order to provide guidance in this respect, an empirical chart
has been developed, which, on the basis of engineering judgment
and experience, provides for me needed degree of compaction in
terms of the climatic rating (Cw) and PI of the soil. Figure 22,
p. 128, graphically portrays this empirical relationship. A varia-
tion of 2% from the optimum density should be considered the maxi-
mum acceptable. However, since soils compacted to 80% of ASTM
D698-63T or 75% of D1557-64T (or latest editions thereof) will
have a relatively low qu, such materials, when used in a fill, should
be subjected to analysis by a qualified soils engineer for both com-
pressibility and potential heaving effects. The amount of compac-
tion required can best be determined by proper analysis and labo-
ratory tests conducted by a qualified soils engineer.
A simple test that has proven helpful in determining the proper
moisture content for compacting clay soils is to squeeze a sample
OCR for page 128
~ ~o ~o ~
Go
or
(] - o
]o
1~/: 1
~-
- i - - -
(jo0Jaq~ suo!~!pa ~sa4Di Jo
`1~9-`9gla JO 109-8690 W1SV)
/~!SU30 J°~°Jd pJapUa~s JO jua3Jad
UO!~ODd~O: If!
~5
CO
x
a)
~5
I
.C) . -
6O
C.
_
o o
C)
C.
U]
so
o
6q ,
.s" o
a;
·_'
o
:-
o
o X
._
._
o
o
a)
._
._
as
o
so
o
Cot
Cot
o ~
C)
-
~o
a;
C)
._
-
OCR for page 129
SUPPLEMENTARY INFORMATION 129
of the material. The sample should be wet enough to hold together
when squeezed, but not wet enough to squeeze through the fingers.
This is a rough guide and merely an aid in determining satisfac-
tory moisture content for the compacted soil.
Highly organic soils should never be used as foundation soils
for slabs and must be removed from the slab site, since they can-
not be satisfactorily compacted and are extremely compressible
because of the decay of organic matter.
2 .1 Fills 1
Clear distinction should be drawn between an uncontrolled fill as
a slab site, a controlled fill, and a cut-and-fill. Each has its own
peculiarities and needs to be treated accordingly.
2.1.1 Controlled Fills
Controlled fills are those placed under specified conditions and
under qualified supervision and testing. The qualities of such fills
are therefore known, and their properties are generally more uniform.
2.1.2 Uncontrolled Fills
Uncontrolled fills are those which do not meet the control re-
quirement. They are subject to nonuniformity, and their qualities
as foundation support cannot be accurately predicted unless veri-
fied beforehand by appropriate soil investigation methods. The
controlling type of soil to be used for slab selection and/or design
purposes should be determined in such fills by the necessary field
and laboratory tests.
2.1.3 Cut-and-Fill
A cut-and-fill is one in which the soil is excavated from one
area of the slab site and redistributed to another area on the site
in order to provide a level surface. When this type of fill is em-
ployed, it is necessary that the fill and natural soil provide the
same degree of support over the slab site The fill material should
be placed as a controlled fill.
Where cuts and fills involving plastic soil (PI > 15) are proposed
Appendix C, Reference 6, Publications No. 1076 and 1281.
OCR for page 130
130 RESIDENTIAL SLABS ON GROUND
as support for structures, the services of a qualified soils engineer
should be required for analysis, testing, and placement supervision.
2 .1.4 Construction of Fills 1
To ensure uniformity, fills should be constructed in shallow in-
crements which are tested adequately in order to eliminate the
possibility of loose layers.
2.1.5 Fill Materials
Where site conditions require fill materials, such fill should
conform to one of the following conditions:
a. Clean granular type such as gravel, crushed stone, or sand,
uniformly graded with 2 - inch maximum size.
b. Any clean fill material (soil free of organic material and
rubbish) installed under competent supervision with mechanical
equipment.
2.2 Natural Moisture Control
Variation in moisture content is an important contributor to soil
behavior; therefore, every effort should be made to eliminate situ-
ations which contribute to variation. With expansive soils, the wa-
ter content required during placement needs to be maintained until
foundations (slabs) are placed. Whereas nothing can be done to
control the occurrence of the natural elements, much can be done
to limit their effect on soil behavior.
2.3 Site Drainage
Unless proper drainage is provided for the slab site, free surface
water will accumulate around and under the slab. This water will
tend to consolidate silts and coarse-grained soils and cause expan-
sive soils to increase in volume. Precautions are necessary to
provide positive drainage away from the slab perimeter, in order
to maintain the best possible state of moisture equilibrium. The
bottom of the surface slab should be a minimum of 6 inches above
the surrounding outside finished grade. The ground should be sloped
1See footnote 2, p. 129.
.N
OCR for page 131
SUPPLEMENTARY INFORMATION 131
down and away from the edge of the slab for 25 feet at a 2% slope,
or a minimum drop of at least 6 inches.
3.0 SLAB MATERIALS
The quality of materials to be used in slab construction is of key
importance. While the quality of materials used in concrete is
generally taken for granted, attention should be directed to the
minimum specifications for materials upon which these recommen-
dations are based.
The latest revisions of the following American Society for Test-
ing and Materials Standards should be used for slab materials:
ASTM C-33 - Specifications for Concrete Aggregates
ASTM C-330 - Specifications for Lightweight Aggregates for
Structural Concrete
ASIM C-150 - Specifications for Portland Cement
ASTM C- 175 - Specifications for Air- Entraining Portland
Cement
ASTM C-94 - Specifications for Ready-Mixed Concrete
ASTM C-205 - Specifications for Portland Blast-Furnace
Cement
ASTM C-260 - Specifications for Air-Entraining Admixtures
for Concrete
ASTM A-15 - Specifications for Billet-Steel Bars for Concrete
Reinforcement
ASTM A-16 - Specifications for Rail-Steel Bars for Concrete
Reinforcement
ASTM A-160 - Specifications for Axle-Steel Bars for Concrete
Reinforcement
ASTM A-185 - Specifications for Welded Steel Wire Fabric
for Concrete Reinforcement.
3.1 Concrete Quality
Mix proportions used should produce a concrete which is workable
and readily finished. Concrete should have low permeability, good
wear and abrasion resistance, and sufficient durability to withstand
local atmospheric and exposure conditions during construction. It
OCR for page 132
132 RESIDENTIAL SLABS ON GROUND
is generally possible to obtain these characteristics in a concrete
having a compressive strength of 2500 psi when tested in accord-
a~ce win ASTM Designation C 39.1
For Type II and IV slabs, the compressive strength, when tested
in accordance with ASTM C-39, should be in accordance with the
ultimate strength used in the structural design.
3.2 Test for Concrete Consistency
The consistency of concrete should be measured in accordance
with ASTM Designation C-143.2
3.3 Admixtures
Admixtures should be permitted when they contribute materially
to one or more of the following in fresh or hardened concrete (and
provided the benefits derived do not entail adverse effect on other
concrete properties recommended in this report): workability,
placeability, ease of finishing, strength, durability, lowered absorp-
tion or permeability, increased abrasion resistance.
4.0 CONCRETE PRAC TIC ES
Placement practices control to a very great extent the service-
ability of any slab. To this end, such practices have been related
to levels of technical competence in proportioning and mixing which
may be prodded on the job.
4.1 Engineer or Architect Supervision
Where proportioning, mixing, and placing of concrete are performed
under control of a competent representative of Me architect or
1ASTM Designation C-39, Method for Test for Compressive Strength
of Molded Concrete Cylinders. Philadelphia: American Society for Testing
and Materials.
2ASTM Designation C-143, Method of Test for Slump of Portland-Cement
Concrete. Philadelphia: American Society for Testing and Materials.
OCR for page 133
SUPPLEMENTARY INFORMATION 133
engineer, the following ACI Standards should be considered to
apply: 1
1. Recommended Practice for Selecting Proportions for Con-
crete (ACI 613)
2. Recommended Practice for Selecting Proportions for Struc-
tural Lightweight Concrete (ACI 613A)
3. Recommended Practice for Measuring, Mixing, and Placing
Concrete (ACI 614)
4. ACI Manual of Concrete Inspection (1957~.
4.2 Ready-Mix Concrete without Engineer or Architect Supervision
Where proportioning, mixing, and placing of ready-mix concrete
are not under the control of an architect or engineer, the following
criteria should be considered to apply:
a. Concrete, with or without admixtures, should equal or sur-
pass-in strength, durability, impermeability, and ease of finish-
ing-plain concrete having a ratio of not more than 6 gallons of
water per sack of cement.
b. The slump of the concrete as designed and as placed on the
job should be not less than 3 nor more than 6 inches; for light-
weight concrete, the slump should be 1 to 3 inches when tested in
accordance with ASTM Designation C-143.
c. The contractor should submit to FHA signed delivery tickets
for ready-mix concrete, attesting to compliance with the specifi-
cations.
d. Under some conditions, such as hot, dry weather with long
waiting periods between mixing and placing of concrete, it may
be necessary to add water to the mix at the job site to regain rec-
ommended slump after water has evaporated from the mix. In such
instance (in order not to affect the quality of concrete), only suffi
1Separate publications of the American Concrete Institute, Detroit,
Michigan.
OCR for page 134
134 RESIDENTIAL SLABS ON GROUND
cient water should be added to regain the specified consistency,
provided the water-to-cement ratio is not exceeded.
4.3 On-Site Mixing
Where concrete is to be mixed on a volumetric basis, and not under
the control of a competent representative of the architect or engi-
neer, the proportions in Table V, p. 135, should be used as a guide.
The contractor should submit signed statements to FHA, attesting
to compliance with Table V. The statement should show total gal-
lons of water and cubic feet of coarse and fine aggregate used for
each sack of cement.
4.4 Concrete Placing and Finishing
The concrete should be distributed and placed in such a manner
that it will work readily into corners and angles of forms, and
around reinforcement, without permitting materials to segregate
or allowing excess free water to collect on the surface. After
placing, concrete should be thoroughly consolidated by spading or
vibration, after which it should be screeded to proper grade. Ex-
cessive spading or vibration should be avoided, to eliminate risk
of separation of materials. The concrete should be worked with a
float to remove high spots and to fill depressions. After floating,
various finishes can be applied according to standard practices.
Where a smooth, troweled finish is required, the surface should
be sufficiency hard not to be marred by the weight of the machine.
4.5 Curing Practices
Curing should commence as soon as concrete has set sufficiently
to prevent damage to the surface. To prevent too rapid drying, the
concrete slab should be covered as soon after placement as is
possible without marring the surface.
Materials such as moist burlap, canvas, cotton matting, liquid
membranes, foaming compounds, polyethylene sheeting, or water-
proof curing paper with edges sealed, may be used to cover the con-
crete during curing. If burlap, canvas, or cotton material is used,
cover material should be kept moist. At no time during the curing
period should concrete be exposed directly to the drying actions
of sun or wind.
OCR for page 135
x
· -
o
v
He
v
a) c.)
cS it,
.
o CQ
Q4
¢
v
o
3
U:
3
Cal
GO
.~-
s"
o
a,
x
sit
¢
be
sol
be
¢
q)
~ N
in
. -
cd
·_.
-
o
1
GO
X
._
C)
o
C)
~3
¢
~ .=
a) ,=
all be
`4 ~
O it,
. - lo,
_
· - ~
Cd ~
CO ~ Cal
. . .
U~
di
l
d4 ~4 di
~ =' U:
ct hD
O ba
V ¢
U] ¢
a)
C)
-
C~
c~ ~
d4
-
c~
o
o
o
· - l
-
cO
· - ~
c~
· -
:>
s~ ~
~q -
·= 3
~o
o ~
C.
X
s~
a
3
· cd
_ _
_
O ~
Q4
8 ~ ~ _ S -
= ~ 3~
5= ~ ~ ~ ~ ~ .=
S : ~ ~-3
O ~ ~ ~ ~ ·~ ~ O ~
,, ; ~ 2 a~ ~ ~- ~ ~ 5, ~
~ O ~ ~ ~ ~ ~ ~ .m
S I , ~ ~ ~ ~ ~ s0
5 0 ~; _ ~ 3
m~ ~ s~ a) ~
h C) I-~ - -
O O O
C) C) 4= ~Q
di
~;
o
-
.-
a)
-
4=
·
Representative terms from entire chapter:
expansive soils
